This invention relates to a new process for the production of polyisocyanates. The present invention also relates to the compounds obtainable by this process and to their use in the production of polyurethane plastics or production of the corresponding polyamines.
In the production of polyurethane plastics from polyisocyanates and polyhydroxyl compounds, the so-called prepolymer or semiprepolymer process is an important alternative. In this process, a prepolymer containing free isocyanate groups or a semiprepolymer (a mixture of prepolymer with excess monomeric starting polyisocyanate) is prepared in a first stage from excess polyisocyanate and the polyhydroxyl component. In a second stage, the prepolymer is reacted with a chain-extending agent to form a high molecular weight polyurethane (cf. for example High Polymers, Vol. XVI "Polyurethanes", Part I, by J. H. Saunders and K. C. Frisch, Interscience Publishers (1962), or Kunststoff-Handbuch, Vol. VII, "Polyurethane" by R. Vieweg and A. Hochtlen, Carl Hanser Verlag, Munich (1966)).
One disadvantage of this known process is that production of the NCO-prepolymers or NCO-semiprepolymers requires a separate reaction step in which some of the NCO-groups in the starting polyisocyanate are reacted with the alcoholic hydroxyl groups of the hydroxyl component.